Method for improving the target position vector accuracy of an amti radar

ABSTRACT

Target bearing angle accuracy is improved through increased time on target followed by processing including averaging and synthetic aperture techniques. The course target position vector provided by an AMTI interferometer type radar is analytically extended to its point of minimum range. The target range, bearing, and velocity values for the point are adjusted to incorporate a minimized error function derived from measured and calculated target position data for a given interval of time. The adjusted values are used to establish a more accurate target position vector.

United States Patent Sletten et al.

METHOD FOR IMPROVING THE TARGET POSITION VECTOR ACCURACY OF AN AMTIRADAR inventors: Carlyle J. Sletten, Acton; F. Sheppard Holt,Winchester, both of Mass.

Assignee: The United States of America as represented by the Secretaryof the Air Force, Washington, DC.

Filed: Nov. 23, 1971 Appl. No.: 201,581

[56] References Cited UNITED STATES PATENTS 3,161,873 12/1964Hollingsworth ,.343/7.7

Primary ExaminerT. H. Tubbesing Attorney-Harry A. Herbert, Jr. andWillard R. Matthews, Jr.

[57} ABSTRACT Target bearing angle accuracy is improved throughincreased time on target followed by processing including averaging andsynthetic aperture techniques. The course target position vectorprovided by an AMTI interferometer type radar is analytically extendedto its US. Cl ..343/7.7, 343/5 CM point of minimum range. The targetrange, bearing,

and velocity values for the point are adjusted to incor- Int. Cl. ..G01s9/42 porate a mmlmlzed error l derived from sured and calculated targetposition data for a given interval of time. The adjusted values are usedto Field of Search ..343/5 CM, 7.7, 17.1 R establish a more accuratetarget position vector.

1 Claim, 2 Drawing Figures 6 macmrr 20 I9 w 1 amsmui VELOCITY n annexGm- DELAYS CONTROLLED F RE TR AG (F) "M (1) MR n MQTRIX Movmc. CLu'r'TERLOCAL Renee *cpmcge zg cscltm-rn '5 6 ATS Cunmuns Fl-B S 17 l RANGE 6 1511+ 1 l RNTENNA VELOC -r-r A To TRACKER J \SoLn'roR bovrLiil FIXED TR 5TRRJZCTORY RMPMFIER MxER flNGULRfl m' flflsE FREGUEW-Y DELAY AMP SWITCHPREDrc-rol 3511mm JNVERTER FILTERS 1 R s E Pu L551! 2, Movws CLUT'T'ER awe: fi-CQNCILLER 2y. GATE CunwznsB-CT filunumz I2 I F V c VELocwrYnovvcck 1R5 TR L s Cou-riuu.zn Fnfiqum 6149mm MWER n p s CH A e C) M (#1hfwl'r'cH FILTERS M V w a-rn 1): 20 19 u,

METHOD FOR IMPROVING THE TARGET POSITION VECTOR ACCURACY OF AN AMTIRADAR BACKGROUND OF THE INVENTION This invention relates to airbornemoving target indicating radar systems, and in particular to means forimproving the accuracy of target bearing information derived therefrom.

An AMTI radar system typical of the type to which the present inventionapplies is disclosed in our copending U. S. patent application Ser. No.201,5 82 entitled, AMTI Radar Clutter Cancelling Method and Apparatus,filed on even date herewith. Such a system employs synthetic aperturetechniques and utilizes multiple antennas disposed along the flightvelocity vector of the observer aircraft. The target bearing dataprovided by such a system is basically the output of an interferometerwhose accuracy is in the order of 1 to 5. With such a system bearingaccuracy is maximum at minimum target range and diminishes as distancebetween aircraft and headings change. For many applications such coursebearing information is grossly inadequate. There currently existstherefore the need for improving the accuracy of AMTI radar bearinginformation whereby target position vectors can be reliably determinedand projected. The present invention is directed toward satisfying sucha need.

SUMMARY OF THE INVENTION The observer aircraft flight velocity vectorand the target flight velocity vector as described in the interferometerof the AMTI radar are initially extended to a minimum range condition(maximum bearing accuracy condition). Over an interval of time a seriesof position measurements are taken at times 1 t t The target positionfor the time t,, is calculated on the basis of the range, bearing andvelocity values as extended to the minimum range condition. An errorfunction is derived from the difference between the calculated targetposition at t and the actual measured target position at t The errorfunction is minimized with respect to the minimum range conditionparameters and such minimum range condition parameters are subsequentlyadjusted to incorporate the minimized error function. Target bearing isthen calculated using the adjusted minimum range condition parameters.

It is the principal object of the invention to provide a new andimproved method for improving the target position vector of an AMTIradar.

Such object, together with other features and advantages of theinvention, will become more readily apparent from the following detaileddescription when taken in conjunction with the illustrated embodimentsin the accompanying drawings.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of am AMTI radarsystem of the type comprehended by the invention; and

FIG. 2 illustrates relative observer aircraft and target flight velocityvectors and other relevant parameters necessary to resolving targetposition vector problems by the method of the invention.

DETAILED DESCRIPTION OF THE PREFERRED METHOD Referring now to FIG. 1,there is illustrated thereby in block diagram an AMTI radar system ofthe type to which the method of the present invention can advantageouslybe applied. Antennas A, B and C are positioned on the observer aircraftin a line coincident with its velocity vector V a distance S apart. TRswitches l 1 permit transmitting and receiving on all antennas althoughin practice transmitter 10 would normally only transmit through antennaB. The received doppler echo signals are amplified by preamplifiers l2and mixed with the output of local oscillator 13 in mixer 14. Afterfurther amplification by amplifiers 15 the signals are filtered intonarrow band channels by doppler filters 16. Antenna B is provided with afixed delay 17 to permit phase coincidence of the signals on allantennas (both leading and lagging). Doppler frequency signals in theclutter frequency spectrum are delayed on antennas A and C by means ofdelay lines 19. Velocity controlled switch matrixes 18 connect thenarrow band channels to appropriate delays in response to aircraftground speed. The doppler echo signals in the clutter frequency spectrumfrom all antennas are thus made phase coincident. The signals fromantenna B are phase inverted by phase inverter 21. This phase invertedsignal is then summed with the outputs of antennas A and C in cluttercancellers 22. Signals from moving targets are not cancelled and are fedto moving range gate 23. The outputs of range gates 23 are integrated byintegrators 24 to improve system gain. The outputs of integrators 24 arecompared by phase comparator 25 to ascertain target bearing information.The method of the present invention improves the accuracy of suchbearing information in the manner hereinafter described.

The observations on distant or weak echoes require a second or so fordetection (integration in moving range bins) and then the signalsgathered simultaneously by interferometer AB-BC are phase compared andpresented to on-board computer logic for location of targets, theirpresentations, and prediction of future target trajectory. The observeddata are essentially corrected for the aircraft radar platform motion,and analytic rendering of the information as time progresses permitsreviewing of the target over a long base line. This horizontal base lineallows improved angle determination.

The observational data includes relative time, range, range difference,doppler frequency shift (leading to radial range rate), observer (radarplatform) velocity, and bearing angle. Relative time, range dopplerfrequency shift, and observer velocity can be accurately determinedwhile bearing angle can only be coarsely determined to within 1 5. Rangedifferencescan be determined to very high accuracy (-A/50). Bearingangle accuracy can be improved in accordance with the method of theinvention through increased time on target followed by processingincluding averaging and synthetic aperture techniques. The remarkablygood bearing accuracy obtained in the fixed target case with state ofthe art fixed target detection systems depends on accurate determinationof range differences by an observer whose motion over an extended pathis accurately known relative to the target. In the moving target casewith the system presented here the motion of the observer relative tothe target is known only through the coarse (AB-BC interferometer)bearing data. However, the accuracy of the determination of this pathcan be improved by averaging or by least mean square techniques. For thefixed target system the highest angular accuracy for a given length ofobservation interval occurs when the observer path is nearly normal tothe radial vector from the fixed target to the observer. For the movingtarget case the highest accuracy for a given length of observationinterval occurs when the path of the observer relative to the target isnearly normal to the radial vector from the target to the observer. Forboth types of targets these optimum conditions are associated with lowdoppler shift but high rate of doppler shift. They are also associatedwith minimum target range and maximum rate of change of true bearingangle. Thus judicious choice of the observer ground path can lead to anoptimum or nearly optimum observer path relative to the target.

A least mean square method for improving angular accuracy in the movingtarget case proceeds as follows:

Assume that an observer (the radar aircraft) and a target are bothmoving with uniform velocity in a horizontal plane. lf VI, is theobserver radar velocity relative to the ground and 7 is the targetvelocity relative to the ground, then 72,, the velocity of the targetrelative to the observer, is also uniform. If 7:, is accurately knownthen calculation of VI, will yield since Let xy be a rectangularcoordinate system moving in the horizontal plane with origin fixed atthe observer as illustrated in FIG. 2. Then the target position vectorrelative to 0 will be 7=z +2, (tt,,) where t is time, 7; is the targetposition vector to the point on the target path closest to O and t,, isthe time at which r =7;

Let

r,, magnitude of F;

0,, angle of 7,, measured positive counter clockwise relative to the xaxis,

and v,, magnitude of 7:, with sign affixed positive if target motionabout 0 is counterclockwise and negative if target motion is clockwise.The vectors F: and VI, can then be written i r,, r,, cos 0,, H-r sin 0,;

0 4 v,, v,,s1n 0,, H-v cos 0,,

A A u where i and j are the conventional umt vectors in the x and ydirections respectively.

The target position vector is then a function of the four parametersr,,, 0,,, v,,, and t,, as follows:

+ [r,, sin 0,, (t t,,) v,, cos 0,, ]j

Let the position of a target be observed to within a known accuracy at aseries of times t 2,, t I Assuming the target to have uniform velocity,we wish to determine r,,, 0,,, v,,t,, such that the target motionassociated with the choice of the parameters fits the observed data mostclosely in a least mean square sense. Let the error function E bedefined as A where p,, x l y,, j is the observed target position at tt,, and r,, is the target position calculated at z t,, on

the basis of general target parameters, r 0,,,V,, and t,,. The procedureis to minimize E relative to these parameters.

Necessary conditions for minimizing E are SE/Br 8E/80 8E/u5v 6E/8t 0These conditions lead to the equations:

[56,, cos 0 y sin 0 -11,] =0

[I sin 0 y cos 00+ no) ol= X sin 0,, Y cos 0,, (T Nt,,)v,,

where N N X z I X.=E t r 11:1 n=l N N Y E 1/", 1 2 n?! 1 =1 N N 2 n, r=2n i n=l Also sin 0,, cos 0,, 1

implies o) o) 0 X +X Substituting (5) and (6) into (3) yields 5Substituting (1) into (2) yields +22%) Sin 0+ -2 +21%) 005 o= 0 o 0 andsubstituting (5) and (6) into (9) yields rr+rw--ar rzow w Equations (7),(8) and 10) can be solved for r,,, t and v, as follows:

r, A B /(BN) (CN AT) While the invention has been described in terms ofone presently preferred method, it is understood that the words whichhave been used are words of description rather than words of limitationand that changes with the purview of the appended claims may be madewithout departing from the scope and spirit of the invention in itsbroader aspects. What is claimed is:

1. In conjunction with an observer aircraft equipped with an AMTI radarsystem capable of providing target range and course interferometerbean'ng data, the method of deriving a target position vector ofimproved accuracy comprising the steps of assigning values for theparameters of minimum range, bearing at minimum range, target velocity,and time at which the target is at minimum range for the true targetpath,

analytically extending a course target velocity vector obtained from theradar system output to a minimum range condition,

taking a series of target position measurements at times t t calculatingon the basis of the extended range, bearing and velocity values atminimum range condition the target position at time t,,, deriving anerror function from the difference between the final measured targetposition values and the calculated target position values, minimizingsaid error functions for range bearing and velocity values at theminimum range condition, adjusting the extended range, bearing andvelocity values for minimum range conditions to incorporate saidminimized error functions, and calculating a target position vector fromsaid adjusting values.

1. In conjunction with an observer aircraft equipped with an AMTI radarsystem capable of providing target range and course interferometerbearing data, the method of deriving a target position vector ofimproved accuracy comprising the steps of assigning values for theparameters of minimum range, bearing at minimum range, target velocity,and time at which the target is at minimum range for the true targetpath, analytically extending a course target velocity vector obtainedfrom the radar system output to a minimum range condition, taking aseries of target position measurements at times t1, t2, . . . tn,calculating on the basis of the extended range, bearing and velocityvalues at minimum range condition the target position at time tn,deriving an error function from the difference between the finalmeasured target position values and the calculated target positionvalues, minimizing said error functions for range bearing and velocityvalues at the minimum range condition, adjusting the extended range,bearing and velocity values for minimum range conditions to incorporatesaid minimized error functions, and calculating a target position vectorfrom said adjusting values.